Two-way carrier wave system



Oct. 20, 1936. H. WHlTTLE TWO-WAY CARRIER WAVE SYSTEM Filed Dec. 28, 1934 E v w 02 E536 mo qxstmmk INVENTOR .H. WH/TTLE ATTOP 'Y Patented Oct. 20, 1936 PATENT or-Flor.

TWOrWAY CARRIER WAVE SYSTEM Horace Whittle, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application Decembe'r;28, 1934, Serial No. 759,436 1 5 Claims. (01. 179-15) A .further object is to provide amplification of 1 transmitted or received waves in a system using the same modulator-demodulator for sending and '10 receiving and the same frequency band for both sending and receiving.

The existence of practical forms of modulatordemodulator or modem circuits, such as those using copper-oxide or other non-linear resistance \devices, suggests the desirability of a carrier terminal using one or several two-way channels I each on a two-wire basis rather than on the usual four-wire basis. In such a system each two-wire voice line would contain a modem unit through which the transmitted and received waves pass in opposite directions, thus avoiding the cost and complexity of hybrid balancing circuits or other equipment necessary for separating outgoing from incoming waves for the purpose of modulation and demodulation.

A difliculty arises in such simplified terminal circuits, however, when attempt is made to introduce amplification. Even if the main carrier lines are operated on a four-wire basis, it is very diflicult to use terminal amplifiers if a high frequency hybrid balance is required. In a multiplex system, for example, the requisite balance to secure a practical degree of amplification would involve balancing some kind of a network against a number of bi-directional channels each including a modem unit and band filter, at best a costly and difiicult procedure.

In accordance with the present invention the necessity of line balance is avoided by the use of 40 terminal switching circuits associated with each channel for rendering the channel terminal unidirectional for either transmitting or receiving, under control of waves sent through the system. A better understanding of the invention and its various features and objects will be had from the following detailed description in connection with the accompanying drawing, of which the single figure shows a schematic circuit diagram of a terminal of a multiplex carrier current system in accordance with the invention. In the drawing two main multiplex carrier lines LE and LW are shown connected to the west terminal. It will be understood that a similar terminal is connected to these two lines at the east station. The line LE transmits a number of duplicate of filter 5, and a resistance network 8. The transmitting and receiving branches. are

side-bands eastward from the terminal circuit shown and includes a common amplifier 21 for amplifying all of the side-bands transmitted over the line LE. Line LW transmits a number oi I side-bands westward and includes an amplifier 28. 5

A number of low frequency lines L1, L2, etc. are assumed to lead up to the terminal circuit shownin the drawing, these lines being voice frequency lines of any suitable type. Each such voice line is equipped with modulating and demodulating 10 apparatus and is arranged for connection to both of the main lines LE and LWin a manner now to be described. I

Considering the line L1 as typical, this line includes a low-pass filter I, a modulator-demodul5 lator or modem 2 supplied from a carrier source 3 and a band filter 4. The modem unit 2 is any suitable type of bi-lateral modulator-demodu lator, one example being a circuit of bridge configuration including copper oxide rectifiers in 20 the four arms of the bridge as disclosed, for example, in United States Patent 1,959,459 to F.

A'. Cowan, granted May 22, 1934, or in an application for' United States patent of R. S. Caruthers, Serial No. 723,691, filed May 3, 1934. The 25 hand filter 4 is selective of one of the two sidebands produced by the modem unit and preferably is sufiiciently long or is otherwise constructed to give an appreciable delay to the waves transmitted through it. v 30 The terminals of the band filter 4 remote from the modem 2 are connected to each of the lines LE and LW by connection respectively to the bus conductors IB and IQ for these two lines. The transmitting branch leading from the band 35 filter 4 includes resistance network 9 and a portion of filter 5 selective to the same side-band as filter 4. This filter 5 is indicated as comprising but a single filter section which prevents channel I from dissipating the-energy of channels 2, 3 0

etc. by ofiering high impedance to the currents utilized by these latter channels. The receiving branch leading from line LW to the band filter 4 includes a single section filter 6 which may be a rendered alternately operative by means of a switching circuit shown in the form of polarized relay 1, the armature l4 of. which normally 50 short-circuits the shunt resistance of networkvil and the armature l3 of which is adapted when w attracted to short-circuit the shunt impedance when the circuit is either idle or in receiving condition, is that shown in the drawing.

It will be understood that the other lines La, etc. are similarly equipped with terminal apparatus, that for the line L: being generally indifor communication with other voice frequency lines, there being no special significance in the number of channels indicated.

The switching apparatus associated with each channel will be better understood from tracing through an operational cycle in connection with the switching vcircuit shown for channel I. As stated, this circuit is shown in the normal or receiving condition. In this condition, the armatures l3 and I4 of polarized relay 1 are held in their retracted position by the aid of biasing winding l8 shown connected to a suitable battery and variable resistance which may be used to regulate the strength of the bias current.

When speech is received over the voice line Ll it passes through low-pass filter I into modem unit 2 where it modulates the carrier wave from the source 3. One of the side-bands, for example the lower side-band resulting from such modulation, is selectively transmitted through the band filter 4 which, as stated, is constructed to introduce a slight delay in this side-band. Some of the output from modem 2 passes into circuit 26 and is impressed on the amplifier-rectifier [0, the rectified output of which flows through the winding ll of the relay 1 and causes that relay to attract its armatures l3 and I4. The single section filter 25 is preferably used to make the rectifier l responsive to the side-band that is not transmitted over the line, that is, responsive to the upper side-band, in this case.

It is assumed that the amount or delay introduced into the side-band'waves by the filter l is substantially that required for the relay 1 to operate, or at least to cause the separation of armature M from its back contact. The breaking of the short circuit at the armature ll around the shunt element of resistance network 9 allows the side-band current selected by the filter 4 to be transmitted through the network 9 and thence through filter to amplifier 21 and line LE. Some of the side-band currents on the output side of the network 9 pass into the amplifier-rectifier H, the rectified output of which flows through the winding l5 of the relay I in a direction to aid the winding I! to hold the armatures l3 and H of the relay I in their attracted position. The relay I remains energized as long as voice currents continue to come in on the line L1. When a substantial pause is made by the talker on line L! current flow ceases in the windm'gs l6 and ll of the relay 1 and the armatures l3 and u of the relay are allowed to tail back after a suitable hang-over period.

Assuming that no speech is coming from the talker on line L1 but that a talker on the voice line associated with the distant terminal, channel I, of the lines LE and LW starts to talk, this results in sending out from the distant terminal on the line LW 9. voice side-band in a manner similar to that described above in connection with the transmission from line L1 into the line LE. This side-band as received over the line LW is amplified at 28 and is selectably transmitted through band filter 6. Part of the energy of this side-band passes into amplifier-rectifier I2, the rectified output of which flows through winding (5 of relay I which is so poled as to aid the bias winding [8' in holding'the armatures l3 and It in a retracted position. The received side-band passes through network 8 and band filter 4 to the modem 2 where it is demodulated with carrier waves from the source 3 and the resulting low frequency or voice frequency wave passes through the low-pass filter I out on to the line Ll. One.purpose for the selective network 25 in the :branch circuit 26 is to prevent an appreciable amount of the received side-band from finding its way into amplifier-rectifier ID. The fact that one side-band only of the modulated wave is transmitted whereas both side-bands are produced by the modem 2 upon transmitting provides a margin of selectivity between outgoing and incoming waves for control of amplifier-rectifier l0. Even if this selectivity margin were not present, however, it will be noted that the rectified currents from amplifier-rectifier l0 flowing through winding H are opposed by currents in both the windings l5 and I8 and they would, therefore, be ineffective in causing attraction of the armatures I3 and [4. The amplifier-rectifier H is maintained unaffected by the received side-band so long as the armature l4 maintains a short circuit in shunt of the branch to which amplifier-rectifier ll isconnected.

If the party on the line L1 should start to talk while speech is in progress in the opposite direction over the circuit, he will be unable to obtain control of the switching circuit since the relay 1 maintains its armatures in their retracted position by the combined efiect of current in windings l5 and I8 which are strong enough to predominate over current flowing in the single winding II.

If the circuit is in the transmitting condition, that is, with relay 1 operated to attract its armatures I3 and M by virtue of speech coming in on line L1, the party at the distant terminal is unable to gain control of the switching circuit at the terminal shown since in that case current is flowing in both windings l6 and I! in the manner described above; the effect of current in the winding l1 predominates over the current in bias winding l8; and the efiect of current in winding l6 offsets that of current in winding l5. Moreover, the efiectiveness of the winding I5 is reduced by virtue of the short circuit across the shunt impedance of network 8 on account of the attraction of the armature l3.

The single section filters 5, 6, 20 and 2|, etc. are used adjacent the high frequency line to prevent disturbances in the parallel connected channel circuit by operation of the switching circuit. If it were not for these filters, the operation of relay 1 in placing low impedance shunts across the transmitting and receiving branches of channel I would deleteriously affect the shunt impedance across the terminals of channel 2 and the other channels. Filters 5, 5, etc. offer such a high impedance outside their bands that the change of impedance effected by the switching-operations in any given channel have no efiect on neighboring channels. These terminal sectional filters 5, 6, etc. are not necessarily additional filters but may be the end sections that would ordinarily be present in the channel band filter 4 as that would ordinarily be constructed.

While a multi-winding relay has been shown for accomplishing the switching, it will be under-- 1 claims stood that the invention -is not limited to this.

specific type of apparatus but that any other suitable circuit controlling means may be employed at this point. It will be understood that the invention is not to be construed to the details that have been shown and'described but that the scope is indicated by the appended what is claimed is: 1. In a carrier wave system. twomain lines transmitting modulated carrier waves in respec- 'be carried out between said tiv'ely opposite directions, a voice line terminatingin a two-way modulator-demodulator circuit and band filter, a branch circuit leading "from said band filter to each of said two main lines, and switching apparatus controlled by waves sent through the system for renderingan'amplifier, a carrier receiving circuit containingan amplifier. each of said circuitsbeing operatively associable with said filter-through a switching apparatus. and means for operating said switching apparatus under control oi waves sent through thesystemto operatively associate said circuits one at a time with said filter.

3. In a carrier wave system, an eastward oneway linepa westward one-way line, each line containing a one-way amplifier, means transmitting modulated carrier waves of the same frequency range over each line,"compiising at a terminal common to said lines. a modulator-demodulator unit and a carrier wave filter, a switch for operatively' associating the terminals or saidfilter opposite the terminals that are connectedto the modiflator-demodulator unit. with said lines oneatatime, andmeans acting in response to waves sent through the system for controlling said switch.

v 4. In a'two-way carrier system, a modem -ior modulating carrier waves for transmission and demodulating received waves, a band filter connected to said modem unit for selectively transmitting and receiving one side-band of a modulated wave and suppressing the opposite side-band. acarrier wave transmitting circuit,

a carrier wave receiving circuitl'a switch for operatively associating the terminals or said band filter with said circuits one at a time, means tocontrol saidswitch by energy in said receiving circuit for efiecting association with one or said two circuits and means to control said switch by energy 01' the side-hand that is suppressed by said filter for eiiecting association with the other of said two circuits. I

5, In a multiple! carrier wave system, an eastward line, a westward line, a plurality of twm way lines; a wave modulating and demoduiating q ipment and a band filter associated witheacb two-way line, a sending and a receiving carrier wave branch from each of said band filters to said eastward and westward lines, respectively, switching circuits for rendering said carrier wave branches 'efi'ective one at a'time, and im- I pedance devices in said branches'ior properly terminating said band filters under the difierent conditions of operation or said switching circuits.

HORACI WHI'I'I'LI. 

